In an optical communication system, a transimpedance amplifier is used to convert a current signal generated by a photo detector into a voltage signal. In 4-level pulse amplitude modulation (PAM4) transmission, the conversion from the current signal to the voltage signal is required to have linearity. Therefore, the transimpedance amplifier has a variable gain, which is controlled to prevent amplitude of the voltage signal from being saturated. The amplitude of the voltage signal is monitored for a feedback control of the variable gain.
Japanese Unexamined Patent Publication No. 2004-304775 discloses a variable gain differential amplifier. The variable gain differential amplifier includes a pair of transistors, and a variable resistance circuit which is provided between emitters of the pair of transistors. The variable resistance circuit includes a resistor and a field effect transistor (FET) connected in series with the resistor. In the variable gain differential amplifier, a control voltage is applied to the gate of the FET and resistance between the source and the drain of the FET is varied in accordance with the control voltage so as to perform a gain control.
In the configuration of applying the control voltage to the gate of the FET such as the variable gain differential amplifier disclosed in Japanese Unexamined Patent Publication No. 2004-304775, a resistance value between the source and the drain is varied by using a transition state between an ON state for electrically connecting the drain and the source and an OFF state for electrically disconnecting the drain and the source by applying the control voltage close a threshold voltage of the FET. However, the transition state depends on the terminal voltages of the source and the drain. Rising and falling of the terminal voltages may affect the resistance value between the source and the drain. Therefore, there is a concern that an output signal is distorted with respect to an input signal.
In the present disclosure, there are provided a transimpedance amplifier and a variable gain circuit of which the distortion can be reduced.